Extracellular and whole-cell patch-clamp recordings were made from neurons in the ventromedial preoptic area in rat horizontal brain slices. Responses to single-pulse electrical stimulation of the ipsilateral suprachiasmatic nucleus were characterized using peristimulus time histograms or postsynaptic current recordings, and bath application of neurotransmitter receptor antagonists. Extracellular recordings showed that suprachiasmatic nucleus stimulation (50-150 microA) elicited a short-latency suppression in 35 of 64 neurons (55%), with the majority (29/35, 83%) showing a biphasic response consisting of a short-latency suppression followed by a long-duration activation. In addition, 14 cells (22%) showed activation only, while 15 (23%) were unresponsive. The GABA(A) receptor antagonist bicuculline (5-10 microM) reversibly blocked suppressions evoked by suprachiasmatic nucleus stimulation (20/20 cells). In the majority of these neurons (13/20), bicuculline also unmasked an activation in response to stimulation. Activations elicited by suprachiasmatic nucleus stimulation, either in the presence or absence of bicuculline, were blocked by the non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10/10 cells). 6,7-Dinitroquinoxaline-2,3-dione (10 microM) selectively and reversibly blocked the initial, short-duration (<50 ms) activation, but had no effect on the longer-duration activation. In contrast, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 microM) appeared to inhibit the long-duration activation selectively without affecting the initial rapid activation. Combined applications of the two ionotropic glutamate receptor antagonists blocked stimulation-induced activations completely. All the pharmacological effects were concentration dependent. Whole-cell patch-clamp recordings showed that suprachiasmatic nucleus stimulation elicited inhibitory postsynaptic currents or a combination of inhibitory and excitatory postsynaptic currents in 25 of 33 neurons tested. The inhibitory postsynaptic currents had short onset latencies (4.9+/-0.3 ms) and a reversal potential of -56.0+/-3.8 mV (n=5), and were reversibly blocked by bicuculline (5-10 microM, 4/4 cells). In the presence of bicuculline (5-10 microM), excitatory postsynaptic currents had short onset latencies (4.7+/-0.5 ms), and had a fast and a slow component. (+/-) 3-(2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid blocked the slow, but not the fast, component, whereas 6,7-dinitroquinoxaline-2, 3-dione blocked the fast, but not the slow, component (n=7). These results suggest that the projection from the suprachiasmatic nucleus conveys both inhibitory (GABA) and excitatory (glutamate) inputs to the ventromedial preoptic area. GABA(A) receptor and both non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptors mediate these influences. These inputs may be responsible for conveying information related to circadian phase from the suprachiasmatic nucleus to regions of the preoptic area known to be involved in regulation of sleep/waking and other physiological functions.